After the collapse of the Soviet Union in 1991 and the resultant end of the Cold War, the threat of a major nuclear war between the two nuclear superpowers was generally thought to have declined. Since then, concern over nuclear weapons has shifted to the prevention of localized nuclear conflicts resulting from nuclear proliferation, and the threat of nuclear terrorism.

The possibility of using nuclear weapons in war is usually divided into two subgroups, each with different effects and potentially fought with different types of nuclear armaments.

The first, a limited nuclear war (sometimes attack or exchange), refers to a small-scale use of nuclear weapons by two (or more) belligerents. A "limited nuclear war" could include targeting military facilities - either as an attempt to pre-emptively cripple the enemy's ability to attack as a defensive measure, or as a prelude to an invasion by conventional forces, as an offensive measure. This term could apply to any small-scale use of nuclear weapons that may involve military or civilian targets (or both).[dubious– discuss][according to whom?]

The second, a full-scale nuclear war, could consist of large numbers of nuclear weapons used in an attack aimed at an entire country, including military, economic, and civilian targets. Such an attack would almost certainly destroy the entire economic, social, and military infrastructure of the target nation, and would probably have a devastating effect on Earth's biosphere.

Some Cold War strategists such as Henry Kissinger[10] argued that a limited nuclear war could be possible between two heavily armed superpowers (such as the United States and the Soviet Union). Some predict, however, that a limited war could potentially "escalate" into a full-scale nuclear war. Others[who?] have called limited nuclear war "global nuclear holocaust in slow motion" - arguing that once such a war took place, others would be sure to follow over a period of decades, effectively rendering the planet uninhabitable in the same way that a "full-scale nuclear war" between superpowers would, only taking a much longer (and arguably more agonizing) path to the same result.

Even the most optimistic predictions[by whom?] of the effects of a major nuclear exchange foresee the death of many millions of victims within a very short period of time. More pessimistic predictions argue that a full-scale nuclear war could potentially bring about the extinction of the human race, or at least its near extinction, with only a relatively small number of survivors (mainly in remote areas) and a reduced quality of life and life expectancy for centuries afterward. However, such predictions, assuming total war with nuclear arsenals at Cold war highs, have not been without criticism.[5] Such a horrific catastrophe as global nuclear warfare would almost certainly cause permanent damage to most complex life on the planet, its ecosystems, and the global climate. If predictions about the production of a nuclear winter are accurate, it would also change the balance of global power, with countries such as Australia, New Zealand, India, China, Argentina and Brazil predicted to become world superpowers if the Cold war ever led to a large-scale nuclear attack.[6]

A study presented at the annual meeting of the American Geophysical Union in December 2006 asserted that even a small-scale regional nuclear war could produce as many direct fatalities as all of World War II and disrupt the global climate for a decade or more. In a regional nuclear conflict scenario in which two opposing nations in the subtropics each used 50 Hiroshima-sized nuclear weapons (c. 15 kiloton each) on major population centers, the researchers predicted fatalities ranging from 2.6 million to 16.7 million per country. The authors of the study estimated that as much as five million tons of soot could be released, producing a cooling of several degrees over large areas of North America and Eurasia (including most of the grain-growing regions). The cooling would last for years and could be "catastrophic", according to the researchers.[11]

Either a limited or full-scale nuclear exchange could occur during an accidental nuclear war, in which the use of nuclear weapons is triggered unintentionally. Postulated triggers for this scenario have included malfunctioning early warning devices and/or targeting computers, deliberate malfeasance by rogue military commanders, consequences of an accidental straying of warplanes into enemy airspace, reactions to unannounced missile tests during tense diplomatic periods, reactions to military exercises, mistranslated or miscommunicated messages, and others. A number of these scenarios actually occurred during the Cold War, though none resulted in the use of nuclear weapons.[12] Many such scenarios have been depicted in popular culture, such as in the 1962 novel Fail-Safe (released as a film in 1964), the film WarGames, released in 1983 and the film Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb, also released in 1964.

Mushroom cloud from the atomic explosion over Nagasaki rising 60,000 feet into the air on the morning of August 9, 1945.

During the final stages of World War II in 1945, the United States conducted atomic raids on the Japanese cities of Hiroshima and Nagasaki, the first on August 6, 1945, and the second on August 9, 1945. These two events are the only time nuclear weapons have been used in combat to date.[13]

For six months before the atomic bombings, the U.S. 20th Air Force under GeneralCurtis LeMay executed low-level incendiary raidsagainst Japanese cities. The worst air raid to occur during the process was not the nuclear attacks but the Operation Meetinghouse raid on Tokyo. On the night of March 9–10, 1945, Operation Meetinghouse commenced and 334 B-29 Superfortressbombers took off to raid with 279 of them dropping 1,665 tons of incendiaries and explosives on Tokyo. The bombing was meant to burn wooden buildings and indeed the bombing caused fire that created a 50 m/s wind that is comparable to tornadoes. Each bomber carried 6 tons of bombs. A total of 381,300 bombs, which amount to 1,783 tons of bombs, were used in the bombing. Within a few hours of the raid, it killed an estimated 100,000 people and destroyed 16 square miles (41 km2) of the city and 267,000 buildings in a single night — the deadliest bombing raid in military aviation history other than the atomic raids on Hiroshima and Nagasaki.[14][15][16][17] By early August 1945, an estimated 450,000 people died as the U.S. intensely firebombed a total of 67 Japanese cities.

In late June 1945, as the U.S. wrapped up the two and a half month Battle of Okinawa (which cost the lives of 260,000 people, including 150,000 civilians),[18][19] it was faced with the prospect of invading the Japanese home islands in an operation code named Operation Downfall. Based on the U.S. casualties from the preceding island-hopping campaigns, American commanders estimated that between 50,000-500,000 U.S. troops would die and at least 600,000-1,000,000 others injured while invading the Japanese home islands. The U.S. manufacture of 500,000 Purple Hearts from the anticipated high level of casualties during the U.S. invasion of Japan gave a demonstration of how deadly and costly it would be. PresidentHarry S. Truman realized he could not afford such a horrendous casualty rate, let alone the fact that over 400,000 American servicemen died fighting in both the European and the Pacific theaters of the war.[20]

On July 26, 1945, the United States, United Kingdom, and the Republic of China issued a Potsdam Declaration that called for the unconditional surrender of Japan. It stated that if Japan did not surrender, it would face "prompt and utter destruction."[21][22] The Japanese government ignored this ultimatum, sending a message that they were not going to surrender. In response to the rejection, President Truman issued an executive order ordering the U.S. military to employ atomic bombs against enemy targets. At the time of its use, there were only two atomic bombs available, and despite the fact more were in production back in mainland U.S., the third bomb wouldn't be available for combat use until September.[23][24]

On August 6, 1945, the uranium-type nuclear weapon code named "Little Boy" was detonated over the Japanese city of Hiroshima with an energy of about 15 kilotons of TNT, destroying nearly 50,000 buildings (including the headquarters of the 2nd General Army and Fifth Division) and killing approximately 70,000 people, including 20,000 Japanese soldiers and 20,000 Koreans.[25][26] Three days later, on August 9, a plutonium-type nuclear weapon code named "Fat Man" was used against the Japanese city of Nagasaki with the explosion equivalent to about 20 kilotons of TNT, destroying 60% of the city and killing approximately 35,000 people, including 23,200-28,200 Japanese civilian munitions workers and 150 Japanese soldiers.[27] The industrial damage in Nagasaki was high, partly owing to the inadvertent targeting of the industrial zone, leaving 68-80 percent of the non-dock industrial production destroyed.[28]

Immediately after the atomic bombings of Japan, the status of atomic weapons in international and military relations was unclear. Presumably, the United States hoped atomic weapons could offset the Soviet Union's larger conventional ground forces in Eastern Europe, and possibly be used to pressure Soviet leader Joseph Stalin into making concessions. Under Stalin, the Soviet Union pursued its own atomic capabilities through a combination of scientific research and espionage directed against the American program. The Soviets believed that the Americans, with their limited nuclear arsenal, were unlikely to engage in any new world wars, while the Americans were not confident they could prevent a Soviet takeover of Europe, despite their atomic advantage.

Within the United States the authority to produce and develop nuclear weapons was removed from military control and put instead under the civilian control of the United States Atomic Energy Commission. This decision reflected an understanding that nuclear weapons had unique risks and benefits that were separate from other military technology known at the time.

For several years after World War II, the United States developed and maintained a strategic force based on the Convair B-36bomber that would be able to attack any potential enemy from bomber bases in the United States. It deployed atomic bombs around the world for potential use in conflicts. Over a period of a few years, many in the American defense community became increasingly convinced of the invincibility of the United States to a nuclear attack. Indeed, it became generally believed that the threat of nuclear war would deter any strike against the United States.

Many proposals were suggested to put all American nuclear weapons under international control (by the newly formed United Nations, for example) as an effort to deter both their usage and an arms race. However, no terms could be arrived at that would be agreed upon by both the United States and the Soviet Union.

American and Soviet nuclear stockpiles.

On August 29, 1949, the Soviet Union tested its first nuclear weapon at Semipalatinsk in Kazakhstan (see also Soviet atomic bomb project). Scientists in the United States from the Manhattan Project had warned that, in time, the Soviet Union would certainly develop nuclear capabilities of its own. Nevertheless, the effect upon military thinking and planning in the United States was dramatic, primarily because American military strategists had not anticipated the Soviets would "catch up" so soon. However, at this time, they had not discovered that the Soviets had conducted significant nuclear espionage of the project from spies at Los Alamos, the most significant of which was done by the theoretical physicist Klaus Fuchs. The first Soviet bomb was more or less a deliberate copy of the Fat Manplutonium device.

With the monopoly over nuclear technology broken, worldwide nuclear proliferation accelerated. The United Kingdom tested its first independent atomic bomb in 1952, followed by France in 1960 and then China in 1964. While much smaller than the arsenals of the United States and the Soviet Union, Western Europe's nuclear reserves were nevertheless a significant factor in strategic planning during the Cold War. A top-secret White Paper, compiled by the Royal Air Force and produced for the British Government in 1959, estimated that British bombers carrying nuclear weapons were capable of destroying key cities and military targets in the Soviet Union, with an estimated 16 million deaths in the Soviet Union (half of whom were estimated to be killed on impact and the rest fatally injured) before bomber aircraft from the U.S. Strategic Air Command reached their targets.

We knew the world would not be the same. A few people laughed, a few people cried, most people were silent. I remembered the line from the Hindu scripture the Bhagavad Gita. Vishnu is trying to persuade the prince that he should do his duty and to impress him takes on his multiarmed form and says, "Now, I am become Death, the destroyer of worlds." I suppose we all thought that one way or another."

Although the Soviet Union had nuclear weapon capabilities in the beginning of the Cold War, the United States still had an advantage in terms of bombers and weapons. In any exchange of hostilities, the United States would have been capable of bombing the Soviet Union, whereas the Soviet Union would have more difficulty carrying out the reverse mission.

The widespread introduction of jet-powered interceptor aircraft upset this imbalance somewhat by reducing the effectiveness of the American bomber fleet. In 1949 Curtis LeMay was placed in command of the Strategic Air Command and instituted a program to update the bomber fleet to one that was all-jet. During the early 1950s the B-47 and B-52 were introduced, providing the ability to bomb the Soviet Union more easily. Before the development of a capable strategic missile force in the Soviet Union, much of the war-fighting doctrine held by western nations revolved around using a large number of smaller nuclear weapons used in a tactical role. It is debatable whether such use could be considered "limited" however, because it was believed that the United States would use its own strategic weapons (mainly bombers at the time) should the Soviet Union deploy any kind of nuclear weapon against civilian targets. Douglas MacArthur, an American general, was fired by President Harry Truman, partially because he persistently requested permission to use his own discretion in deciding whether to use atomic weapons on the People's Republic of China in 1951 during the Korean War.[31]Mao Zedong, China's communist leader, gave the impression that he would welcome a nuclear war with the capitalists because it would annihilate what he viewed as their "imperialist" system.[32][33]

Let us imagine how many people would die if war breaks out. There are 2.7 billion people in the world, and a third could be lost. If it is a little higher it could be half ... I say that if the worst came to the worst and one-half dies, there will still be one-half left, but imperialism would be razed to the ground and the whole world would become socialist. After a few years there would be 2.7 billion people again.

The concept of a "Fortress North America" emerged during the Second World War and persisted into the Cold War to refer to the option of defending Canada and the United States against their enemies if the rest of the world were lost to them. This option was rejected with the formation of NATO and the decision to permanently station troops in Europe.

In the summer of 1951 Project Vista started, in which project analysts such as Robert F. Christy looked at how to defend Western Europe from a Soviet invasion. The emerging development of tactical nuclear weapons were looked upon as a means to give Western forces a qualitative advantage over the Soviet numerical supremacy in conventional weapons.[35]

Several scares about the increasing ability of the Soviet Union's strategic bomber forces surfaced during the 1950s. The defensive response by the United States was to deploy a fairly strong "layered defense" consisting of interceptor aircraft and anti-aircraft missiles, like the Nike, and guns, like the Skysweeper, near larger cities. However, this was a small response compared to the construction of a huge fleet of nuclear bombers. The principal nuclear strategy was to massively penetrate the Soviet Union. Because such a large area could not be defended against this overwhelming attack in any credible way, the Soviet Union would lose any exchange.

This logic became ingrained in American nuclear doctrine and persisted for much of the duration of the Cold War. As long as the strategic American nuclear forces could overwhelm their Soviet counterparts, a Soviet pre-emptive strike could be averted. Moreover, the Soviet Union could not afford to build any reasonable counterforce, as the economic output of the United States was far larger than that of the Soviets, and they would be unable to achieve "nuclear parity".

Soviet nuclear doctrine, however, did not match American nuclear doctrine.[36][37] Soviet military planners assumed they could win a nuclear war.[36][38][39] Therefore, they expected a large-scale nuclear exchange, followed by a "conventional war" which itself would involve heavy use of tactical nuclear weapons. American doctrine rather assumed that Soviet doctrine was similar, with the mutual in Mutually Assured Destruction necessarily requiring that the other side see things in much the same way, rather than believing - as the Soviets did - that they could fight a large-scale, "combined nuclear and conventional" war.

A revolution in nuclear strategic thought occurred with the introduction of the intercontinental ballistic missile (ICBM), which the Soviet Union first successfully tested in August 1957. In order to deliver a warhead to a target, a missile was much faster and more cost-effective than a bomber, and enjoyed a higher survivability due to the enormous difficulty of interception of the ICBMs (due to their high altitude and extreme speed). The Soviet Union could now afford to achieve nuclear parity with the United States in raw numbers, although for a time, they appeared to have chosen not to.

Photos of Soviet missile sites set off a wave of panic in the U.S. military, something the launch of Sputnik would do for the American public a few months later. Politicians, notably then-U.S. SenatorJohn F. Kennedy suggested that a "missile gap" existed between the Soviet Union and the United States. The US military gave missile development programs the highest national priority, and several spy aircraft and reconnaissance satellites were designed and deployed to observe Soviet progress.

Early ICBMs and bombers were relatively inaccurate, which led to the concept of countervalue strikes — attacks directly on the enemy population, which would theoretically lead to a collapse of the enemy's will to fight. During the Cold War, the Soviet Union invested in extensive protected civilian infrastructure, such as large "nuclear-proof" bunkers and non-perishable food stores. By comparison, smaller scale civil defense programs were instituted in the United States starting in the 1950s, where schools and other public buildings had basements stocked with non-perishable food supplies, canned water, first aid, and dosimeter and Geiger counter radiation-measuring devices. Many of the locations were given "Fallout Shelter" designation signs. CONELRAD radio information systems were adopted, whereby the commercial radio sector (later supplemented by the National Emergency Alarm Repeaters) would broadcast on two AM frequencies in the event of a Civil Defense (CD) emergency. These two frequencies-640 and 1240 marked with small CD triangles on the tuning dial can still be seen on 1950s-vintage radios on online auction sites and museums. A few backyard fallout shelters were built by private individuals.

In 1960, the United States developed its first Single Integrated Operational Plan, a range of targeting options, and described launch procedures and target sets against which nuclear weapons would be launched, variants of which were in use from 1961 to 2003. That year also saw the start of the Missile Defense Alarm System, an American system of 12 early-warning satellites that provided limited notice of Soviet intercontinental ballistic missile launches between 1960 and 1966. The Ballistic Missile Early Warning System was completed in 1964.

A complex and worrisome situation developed in 1962, in what is called the Cuban Missile Crisis. The Soviet Union placed medium-range ballistic missiles 90 miles (140 km) from the United States - possibly as a direct response to American Jupiter missiles placed in Turkey. After intense negotiations, the Soviets ended up removing the missiles from Cuba and decided to institute a massive weapons-building program of their own. In exchange, the United States dismantled its launch sites in Turkey, although this was done secretly and not publicly revealed for over two decades. Khrushchev did not even reveal this part of the agreement when he came under fire by political opponents for mishandling the crisis. Communication delays during the crisis led to the establishment of the Moscow–Washington hotline to allow reliable, direct communications between the two nuclear powers.

By the late 1960s, the number of ICBMs and warheads was so high on both sides that it was believed that both the United States and the Soviet Union were capable of completely destroying the infrastructure and a large proportion of the population of the other country. Thus, by some western game theorists, a balance of power system known as mutually assured destruction (or MAD) came into being. It was thought that any full-scale exchange between the powers would not result in an outright winner, with at best one side emerging the pyrrhic victor. Thus both sides were deterred from risking the initiation of a direct confrontation, instead being forced to engage in lower intensity proxy wars.

One drawback of the MAD doctrine was the possibility of a nuclear war occurring without either side intentionally striking first. Warning system\Early Warning Systems (EWS) were notoriously error-prone. For example, on 78 occasions in 1979 alone, a "missile display conference" was called to evaluate detections that were "potentially threatening to the North American continent". Some of these were trivial errors and were spotted quickly, but several went to more serious levels. On September 26, 1983, Stanislav Petrov received convincing indications of an American first strike launch against the Soviet Union, but positively identified the warning as a false alarm. Though it is unclear what role Petrov's actions played in preventing a nuclear war during this incident, he has been honored by the United Nations for his actions.

Similar incidents happened many times in the United States, due to failed computer chips,[41] misidentifications of large flights of geese, test programs, and bureaucratic failures to notify early warning military personnel of legitimate launches of test or weather missiles. For many years, the U.S. Air Force's strategic bombers were kept airborne on a daily rotating basis "around the clock" (see Operation Chrome Dome), until the number and severity of accidents, the 1968 Thule Air Base B-52 crash in particular, persuaded policymakers it was not worthwhile.

By the late 1970s, people in both the United States and the Soviet Union, along with the rest of the world, had been living with the concept of mutual assured destruction (MAD) for about a decade, and it became deeply ingrained into the psyche and popular culture of those countries.

On May 18, 1974, India conducted its first nuclear test in the Pokhran test range. The name of the operation was Smiling Buddha, and India termed the test as a "peaceful nuclear explosion".

The Soviet Duga-3 early warning over-the-horizon radar system was made operational in 1976. The extremely powerful radio transmissions needed for such a system led to much disruption of civilian shortwave broadcasts, earning it the nickname "Russian Woodpecker".

The idea that any nuclear conflict would eventually escalate was a challenge for military strategists. This challenge was particularly severe for the United States and its NATO allies because it was believed (until the 1970s) that a Soviet tank invasion of Western Europe would quickly overwhelm NATO conventional forces, leading to the necessity of the West escalating to the use of tactical nuclear weapons, one of which was the W-70.

This strategy had one major (and possibly critical) flaw, which was soon realized by military analysts but highly underplayed by the U.S. military: conventional NATO forces in the European theatre of war were far outnumbered by similar Soviet and Warsaw Pact forces, and it was assumed that in case of a major Soviet attack (commonly envisioned as the "Red tanks rolling towards the North Sea" scenario) that NATO - in the face of quick conventional defeat - would soon have no other choice but to resort to tactical nuclear strikes against these forces. Most analysts agreed that once the first nuclear exchange had occurred, escalation to global nuclear war would likely become inevitable. The Soviet bloc's vision of an atomic war between NATO and Warsaw Pact forces was simulated in the top secret exercise Seven Days to the River Rhine in 1979. The British government exercised their vision of Soviet nuclear attack with Square Leg in early 1980.

Large hardened nuclear Weapon storage area's were built across European countries in anticipation of local US and European forces falling back as the conventional NATO defense from the Soviet Union, named REFORGER, was believed to only be capable of stalling the Soviets for a short time.

In the late 1970s and, particularly, during the early 1980s under U.S. President Ronald Reagan, the United States renewed its commitment to a more powerful military, which required a large increase in spending on U.S. military programs. These programs, which were originally part of the defense budget of U.S. President Jimmy Carter, included spending on conventional and nuclear weapons systems. Under Reagan, defensive systems like the Strategic Defense Initiative were emphasized as well.

Another major shift in nuclear doctrine was the development and the improvement of the submarine-launched, nuclear-armed, ballistic missile, or SLBM. It was hailed by many military theorists as a weapon that would make nuclear war less likely. SLBMs - which can move with "stealth" (greatly lessened detectability) virtually anywhere in the world - give a nation a "second strike" capability (i.e. after absorbing a "first strike"). Before the advent of the SLBM, thinkers feared that a nation might be tempted to initiate a first strike if it felt confident that such a strike would incapacitate the nuclear arsenal of its enemy, making retaliation impossible. With the advent of SLBMs, no nation could be certain that a first strike would incapacitate its enemy's entire nuclear arsenal. To the contrary, it would have to fear a near certain retaliatory second strike from SLBMs. Thus, a first strike was a much less of feasible (or desirable) option, and a deliberately initiated nuclear war was thought to be less likely to start.

However, it was soon realized that submarines could approach enemy coastlines undetected and decrease the warning time (the time between detection of the missile launch and the impact of the missile) from as much as half an hour to possibly under three minutes. This effect was especially significant to the United States, Britain and China, whose capitals all lay within 100 miles (160 km) of their coasts. Moscow was much more secure from this type of threat, due to its considerable distance from the sea. This greatly increased the credibility of a "surprise first strike" by one faction and (theoretically) made it possible to knock out or disrupt the chain of command of a target nation before any counterstrike could be ordered (known as a "decapitation strike"). It strengthened the notion that a nuclear war could possibly be "won" - resulting not only in greatly increased tensions and increasing calls for fail-deadly control systems, but also in a dramatic increase in military spending. The submarines and their missile systems were very expensive, and one fully equipped nuclear-powered and nuclear-armed missile submarine could cost more than the entire GNP of a developing country.[43] It was also calculated, however, that the greatest cost came in the development of both sea- and land-based anti-submarine defenses and in improving and strengthening the "chain of command", and as a result, military spending skyrocketed.

South Africa developed a nuclear weapon capability during the 1970s and early 1980s. It was operational for a brief period before being dismantled in the early 1990s.

According to the 1980 United Nations report General and Complete Disarmament: Comprehensive Study on Nuclear Weapons: Report of the Secretary-General, it was estimated that there were a total of about 40,000 nuclear warheads in existence at that time, with a potential combined explosive yield of approximately 13,000 megatons. By comparison, in the Timeline of volcanism on Earth when the volcano Mount Tambora erupted in 1815 - turning 1816 into the Year Without A Summer due to the levels of global dimmingsulfate aerosols and ash expelled - it exploded with a force of roughly 800 to 1,000 megatons,[citation needed] and ejected 160 km3 (38 cu mi) of mostly rock/tephra,[44] that included 120 million tonnes of sulfur dioxide as an upper estimate.[45] A larger eruption, approximately 74,000 years ago, in Mount Toba produced 2,800 km3 (670 cu mi) of tephra, forming lake Toba,[46] and produced an estimated 6,000 million tonnes (6.6×109 short tons) of sulfur dioxide.[47][48] The explosive energy of the eruption may have been as high as equivalent to 20,000,000 megatons(MT) of TNT,[49][better source needed] while the asteroid created Chicxulub impact, that is connected with the extinction of the dinosaurs corresponds to at least 70,000,000 MT of energy, which is roughly 7000 times the maximum arsenal of the US and Soviet Union.[49]

However, comparisons with supervolcanos are more misleading than helpful due to the different aerosols released, the likely air burst fuzing height of nuclear weapons and the globally scattered location of these potential nuclear detonations all being in contrast to the singular and subterranean nature of a supervolcanic eruption.[7] Moreover assuming the entire world stockpile of weapons were grouped together, it would be difficult, due to the nuclear fratricide effect, to ensure the individual weapons would go off all at once. Nonetheless, many people believe that a full-scale nuclear war would result, through the nuclear winter effect, in the extinction of the human species, though not all analysts agree on the assumptions that underpin these nuclear winter models.[5]

The world came unusually close to nuclear war - although perhaps not as close as during the Cuban Missile Crisis - when the Soviet Union thought that the NATO military exercise Able Archer 83 was a ruse or "cover up" to begin a nuclear first strike. The Soviets responded by raising readiness and preparing their nuclear arsenal for immediate use. Soviet fears of an attack ceased once the exercise concluded without incident.

Although the dissolution of the Soviet Union ended the Cold War and greatly reduced tensions between the United States and the Russian Federation, the Soviet Union's formal successor state, both countries remained in a "nuclear stand-off" due to the continuing presence of a very large number of deliverable nuclear warheads on both sides. Additionally, the end of the Cold War led the United States to become increasingly concerned with the development of nuclear technology by other nations outside of the former Soviet Union. In 1995, a branch of the U.S. Strategic Command produced an outline of forward-thinking strategies in the document "Essentials of Post–Cold War Deterrence".

As a consequence of the 9/11 attacks on the USA, American forces immediately increased their readiness to the highest level in 28 years, closing the blast doors of the NORAD's Cheyenne Mountain Operations Center for the first time due to a non-exercise event. But unlike similar increases during the Cold War, Russia immediately decided to stand down a large military exercise in the Arctic region, in order to minimize the risk of incidents, rather than following suit.[54]

The former chair of the United Nationsdisarmament committee stated that there are more than 16,000 strategic and tactical nuclear weapons ready for deployment and another 14,000 in storage, with the U.S. having nearly 7,000 ready for use and 3,000 in storage, and Russia having about 8,500 ready for use and 11,000 in storage. In addition, China is thought to possess about 400 nuclear weapons, Britain about 200, France about 350, India about 80-100, and Pakistan 100-110. North Korea is confirmed as having nuclear weapons, though it is not known how many, with most estimates between 1 and 10. Israel is also widely believed to possess usable nuclear weapons. NATO has stationed about 480 American nuclear weapons in Belgium, the Netherlands, Italy, Germany, and Turkey, and several other nations are thought to be in pursuit of an arsenal of their own.[55]

A key development in nuclear warfare throughout the 2000s and early 2010s is the proliferation of nuclear weapons to the developing world, with India and Pakistan both publicly testing several nuclear devices, and North Korea conducting an underground nuclear test on October 9, 2006. The U.S. Geological Survey measured a 4.2 magnitude earthquake in the area where the North Korean test is said to have occurred. A further test was announced by the North Korean government on May 25, 2009.[56]Iran, meanwhile, has embarked on a nuclear program which - while officially for civilian purposes - has come under close scrutiny by the United Nations and many individual states.

Recent studies undertaken by the CIA cite the enduring India-Pakistan conflict as the one "flash point" most likely to escalate into a nuclear war. During the Kargil War in 1999, Pakistan came close to using its nuclear weapons in case the conventional military situation underwent further deterioration.[57] Pakistan's foreign minister had even warned that it would "use any weapon in our arsenal", hinting at a nuclear strike against India.[58] The statement was condemned by the international community, with Pakistan denying it later on. This conflict remains the only war (of any sort) between two declared nuclear powers. The 2001-2002 India-Pakistan standoff again stoked fears of nuclear war between the two countries. Despite these very serious and relatively recent threats, relations between India and Pakistan have been improving somewhat over the last few years. A bus line directly linking Indian- and Pakistani-administered Kashmir has recently been established. However, with the November 26, 2008 Mumbai terror attacks, India currently will not rule out war with Pakistan.

Another potential geopolitical issue which is considered particularly worrisome by military analysts is a possible conflict between the United States and the People's Republic of China over Taiwan. Although economic forces are thought to have reduced the possibility of a military conflict, there remains concern about the increasing military buildup of China (China is rapidly increasing its naval capacity), and that any move toward Taiwan independence could potentially spin out of control.

Israel is thought to possess somewhere between one hundred and four hundred nuclear warheads. It has been asserted that the submarines which Israel received from Germany have been adapted to carry missiles with nuclear warheads, so as to give Israel a second strike capability.[59] Israel has been involved in wars with its neighbors in the Middle East (and with other "non-state actors") on numerous prior occasions, and its small geographic size and population could mean that, in the event of future wars, the Israeli military might have very little time to react to an invasion or other major threat. Such a situation could escalate to nuclear warfare very quickly in some scenarios.

In the Persian Gulf, Iran appears to many observers to be in the process of developing a nuclear weapon, which has greatly heightened fears of a nuclear conflict and arms races in the Middle East—either with Israel or with one or more Arab states (a "Shia-Sunni" conflict).

On March 7, 2013, North Korea threatened the United States with a pre-emptive nuclear strike.[60] On April 9, North Korea urged foreigners to leave South Korea, stating that both countries were on the verge of nuclear war.[61] On April 12, North Korea stated that a nuclear war was unavoidable. The country declared Japan as its first target.[62]

The above examples envisage nuclear warfare at a strategic level, i.e. total war. However, nuclear powers have the ability to undertake more limited engagements.

"Sub-strategic use" includes the use of either "low-yield" tactical nuclear weapons, or of variable yieldstrategic nuclear weapons in a very limited role, as compared to battlefield exchanges of larger-yield strategic nuclear weapons. This was described by the UK Parliamentary Defence Select Committee as "the launch of one or a limited number of missiles against an adversary as a means of conveying a political message, warning or demonstration of resolve".[63] It is believed that all current nuclear weapons states possess tactical nuclear weapons, with the exception of the United Kingdom, which decommissioned its tactical warheads in 1998. However, the UK does possess scalable-yield strategic warheads, and this technology tends to blur the difference between "strategic", "sub-strategic", and "tactical" use or weapons. American, French and British nuclear submarines are believed to carry at least some missiles with these types of high-tech warheads for this purpose - potentially allowing a strike as low as one kiloton (or less) against a single target. Only the People's Republic of China and the Republic of India have declarative, unqualified, unconditional "no first use" nuclear weapons policies. India and Pakistan maintain only a credible minimumdeterrence.

Commodore Tim Hare, former Director of Nuclear Policy at the British Ministry of Defence, has described "sub-strategic use" as offering the Government "an extra option in the escalatory process before it goes for an all-out strategic strike which would deliver unacceptable damage".[64] However, this sub-strategic capacity has been criticized as potentially increasing the "acceptability" of using nuclear weapons. The related consideration of new generations of limited-yield nuclear weapons by the United States (i.e. "bunker busters") has also alarmed anti-nuclear groups, who believe it will make the use of nuclear weapons "more acceptable" or likely.

Nuclear terrorism by non-state organizations or actors (even individuals) is a largely unknown and understudied factor in nuclear deterrence thinking, as states possessing nuclear weapons are susceptible to retaliation in kind, while sub- or trans-state actors may be less so. The collapse of the Soviet Union has given rise to the possibility that former Soviet nuclear weapons might become available on the black market (so-called 'loose nukes'). While no warheads are known to have been mislaid, it has been alleged that at least some very small or suitcase-size bombs might be unaccounted for.

A number of other concerns have been expressed about the security of nuclear weapons in other, newer nuclear powers with relatively less stable governments, such as Pakistan, but in each case, the fears have been addressed to some extent by statements and evidence provided by those nations, as well as cooperative programs between nations. Worry remains, however, in many circles that a relative decrease in security of nuclear weapons has emerged in recent years, and that terrorists or others may attempt to exert control over (or use) nuclear weapons, militarily applicable technology, or nuclear materials and fuel.

Another possible nuclear terrorism threat are devices designed to disperse radioactive materials over a large area using conventional explosives, called dirty bombs. The detonation of a "dirty bomb" would not cause a nuclear explosion, nor would it release enough radiation to kill or injure a lot of people. However, it could cause severe disruption and require potentially very costly decontamination procedures and increased spending on security measures.[66]

The predictions of the effects of a major countervalue nuclear exchange include millions of city dweller deaths within a short period of time. Some predictions argue that a full-scale nuclear war could eventually bring about the extinction of the human race; however, such predictions, assuming total war with nuclear arsenals at Cold war highs, have been met with criticism.[5] Nonetheless, a number of Cold War publications advocate preparations that could purportedly enable a large proportion of civilians to survive even a total nuclear war. Among the most famous of these is Nuclear War Survival Skills.[67]

Publications on adapting to a changing diet and supplying nutritional food sources following a nuclear war, with particular focus on agricultural radioecology, include Nutrition in the postattack environment by the RAND corporation.[70]

The British government developed a public alert system for use during nuclear attack with the expectation of a four-minute warning before detonation. The United States expected a warning time of anywhere from half an hour (for land based missiles) to less than three minutes (for submarine based weapons). Many countries maintain plans for continuity of government and continuity of operations following a nuclear attack or similar disasters. These range from a designated survivor, intended to ensure survival of some form of government leadership, to the Soviet Dead Hand system, which allows for retaliation even if all Soviet leadership were destroyed. Nuclear submarines are given letters of last resort; orders on what action to take in the event that an enemy nuclear strike has destroyed the government.

The Soviet government believed they could win not only a strategic nuclear war, which they planned to absorb with their extensive Civil Defense schemes and infrastructure dispersal,[71][72][73] but also the conventional war that they predicted would follow after their strategic nuclear arsenal had been depleted.[74]

A number of other countries around the world have taken significant efforts to maximize their survival prospects in the event of large calamities, both natural and manmade. For example, metro stations in Pyongyang, North Korea, were constructed 110 metres (360 ft) below ground, and were designed to serve as nuclear shelters in the event of war, with each station entrance built with thick steel blast doors.[75][76] An example of a privately funded fallout shelters is the Ark Two Shelter in Ontario, Canada, an autonomous shelters constructed with an emphasis on post-war networking and reconstruction.[77] In Switzerland, the majority of homes have an underground blast and fallout shelter. The country has an overcapacity of such shelters and can accommodate slightly more than the nation's population size.[78][79]

"Presidency in the Nuclear Age", conference and forum at the JFK Library, Boston, October 12, 2009. Four panels: "The Race to Build the Bomb and the Decision to Use It", "Cuban Missile Crisis and the First Nuclear Test Ban Treaty", "The Cold War and the Nuclear Arms Race", and "Nuclear Weapons, Terrorism, and the Presidency".